A massage machine with substantially spherical treatment heads called massage balls driven to trace a three-dimensional trajectory uses, for example, three motors which are driven independently of one another. Treatments such as acupressure, massage and spine stretching are performed by combining an up/down movement along a backrest of a chair, a reciprocal movement in the width direction of the chair (width adjustment) and a revolving movement (back-and-forth movement) of the treatment heads. In recent years, following to complexity of the three-dimensional operations (called hand techniques) of the treatment heads, it is necessary to switch a motor in a short time between a low speed rotation and a high speed rotation, and to frequently switch the motor between a normal rotation and a reverse rotation.
A human body is a load in the massage machine, in which the load significantly varies depending on the figure of a user and the way he or she sits. Furthermore, when the user moves the body during treatment, it causes a weight shift, which leads to significant load change during the treatment. Therefore, a DC motor, particularly a brushed DC motor, is widely used conventionally as a motor which is capable of switching of the rotation speed in a short time and the rotation direction between the normal rotation and the reverse rotation, and which has a high torque. Generally, a high torque brushed DC motor has a feature that it has a large external shape and a heavy weight. Thus, it has been difficult to reduce the size and weight of the massage machine itself. Although there is a massage machine using a small sized brushed DC motor, the motor has a low torque, so that it is difficult to obtain sufficient treatment effects.
On the other hand, since a brushless DC motor is small in size, and superior in maintenance and durability, the use of a brushless DC motor as a driving source for a massage machine has been considered. However, although a brushless DC motor enables to achieve a small sized and high torque (high output) motor, it causes a new problem when used in a massage machine with large load change. More specifically, in the case of using a general 3-phase 4-pole brushless DC motor, the torque significantly varies due to cogging so that acoustic noises of the motor becomes larger if it is driven by a 120-degrees rectangular wave current. Since these noises are transmitted to a human body via the treatment heads, it causes discomfort to a user when a portion near the head such as neck or shoulder is treated. On the other hand, if emphasis is placed on low noise and low vibration, it might be considered to drive a brushless DC motor by sinusoidal wave current. However, it is required to detect the rotation angle of a rotor of the brushless DC motor with high accuracy, and to use an encoder and the like, so that it becomes a factor to prevent reduction in cost, size and weight. Furthermore, in case that a brushless DC motor is controlled by sinusoidal wave driving, it is required to apply currents at intervals of an electrical angle of 180 degrees. However, assuming that a brushless DC motor is used for a massage machine as described above, the sinusoidal wave drive makes it impossible to start or difficult to smoothly start the brushless DC motor because the load is not constant.
In particular, if the motor is not smoothly switched between the normal rotation and the reverse rotation, the treatment heads stop instantaneously, so that the user sensitively recognizes that the treatment heads have stopped. Furthermore, if the motors are driven to allow the treatment heads to trace a predetermined three-dimensional trajectory, it is required to synchronously drive three motors. However, when the rotation speed of an output shaft of a motor (hereafter referred to simply as rotation speed of a motor) deviates from a predetermined speed due to the load change or the like, the treatment heads are driven to trace a trajectory different from the intended trajectory. Thus, there is a possibility that the user cannot obtain a comfortable massage effect.